Method For Locating Oil And Gas Field Boundaries

Abstract

A method and apparatus for detecting the presence, direction of and the distance to barriers in underground oil and gas fields by directional flow data.

Description

This invention relates to a method and apparatus for detecting the presence, and direction to and the distance from the well of barriers or faults in underground oil and gas fields. More particularly, this invention is related to a method and apparatus for locating the presence of oil or gas field barriers by directional flow data.

Conventionally, the presence of and distance to a barrier are detected from reservoir pressure build-up analysis. However, it is not known in the prior art to provide means using pressure build-up analysis to determine the direction of the barrier with respect to the well. If the direction of the barrier is known relative to the well, then the well operator knows in which direction to move to get away from the barrier when drilling subsequent wells. Without this information, during field development, the operator must continue to drill holes on a trial and error basis until the barrier is located. A barrier of this nature establishes the productive boundary of the field.

Pressure build-up in a well in an infinite reservoir will be a straight line relationship between pressure and 1n (t +.DELTA.t)/.DELTA.t wherein t equals the total producing time at producing rate q and .DELTA.t is the length of shut-in time. The slope of the straight line is the expression qu/ckh wherein q is the producing rate, u is the viscosity, c is a constant, k is the permeability and h is the formation thickness. Extrapolation of the curve to 1n (t +.DELTA.t)/.DELTA.t equals zero indicates the static reservoir pressure. At this point on the curve, the producing time t is insignificant compared to the shut-in time .DELTA.t and mathematically, the point is equivalent to infinite shut-in time during which the reservoir would have reached maximum static pressure.

It has been learned, however, that certain pressure variations occur when a well is drilled near a fault or impermeable barrier. In these instances, the curve will not have a single straight line function between pressure and 1n (t +.DELTA.t)/.DELTA.t but a double slop curve will result. The slope of the first part of the curve is identical to that of the build-up relationship in an infinite reservoir, i.e., the slop equals qu/ckh. However, the slope of the second part of the curve is double that of the first part and the slope equals 2qu/ckh. This curve is actually an added pressure rise to the normal pressure rise to the first part of the curve.

If one well is completed in a very large reservoir far from any faults or barriers, a build-up test on this well will give results similar to those that would be obtained if the reservoir were infinite in extent, provided production time is small before the time of the build-up test. The extrapolation of the slope to the zero line 1n (t -.DELTA.t)/.DELTA.t equals essentially the original reservoir pressure.

In a well which is completed near a fault or barrier the pressure during the production period is drawn down and in time the area affected by the reduced pressure will have reached the fault or barrier. Since there is no fluid flow from the direction beyond the fault to replace fluid produced from the area between the well and the fault, there will be a larger radius of pressure disturbance into the reservoir away from the fault than that in the well completed far from any fault or barriers. Therefore, for the same amount of production, the pressure of the well near the fault or barrier will be less than that of the well located in the center of an infinite reservoir.

When a well is shut-in, the pressure build-up is from all directions around the well bore. The initial slopes of a well near a barrier and not near a barrier will be identical, until the interference of the fault barrier is felt in the well close to the barrier. Then the pressure in the latter well begins to build at an additional rate so that the initial slope is doubled as heretofore explained. It is an objective of this invention to utilize this information to detect the presence and more particularly the direction of a fault or a barrier in an oil or gas field when a well is completed near a fault or a barrier.

It is a further objective of this invention to provide a simple and economical apparatus for sensing differential pressures around the periphery of a completed well within the reservoir and for interpreting the differential pressure readings for indicating the presence and direction of a fault or a barrier.

The lowest pressure point of a reservoir during the production and shut-in time of a well completed near the center of a large reservoir is at the well bore. For a well completed near a fault or barrier, the lowest pressure point of a reservoir is at the well bore during the production period. The well bore is also the point of lowest pressure during the first part of the build-up period following shut-in. This period is indicated by the first slope of the curve. The theory on which this invention is based is that when the interference of a fault or barrier begins to affect the pressure, it is because the lowest pressure point in the reservoir is moving away from the well toward the fault. It is an objective of this invention to utilize this theory by sensing the pressure differential across the well bore in the direction of fluid flow across the area of the reservoir containing the well bore. The fluid flow would be caused by the movement of the lowest pressure point away from the central point of the well. The direction of flow is detected by pressure differential sensing apparatus and indicates the direction of the barrier. In the practice of this invention the direction would be perpendicular to the fault or barrier and a zero pressure differential will normally exist across the well bore in a direction parallel to the boundary.

This invention comprises the placement of a plurality of differential pressure gauges within a housing mounted in the well in the gas or oil reservoir. Preferably, the differential pressure gauges are capable of measuring pressure differences of as low as 0.0000001 psi and pressures to 5000 psi. The ends of each pressure gauge communicate with the reservoir by means of a pair of diametrically opposed apertures whereby said gauges sense the differential pressure at the apertures of each pair. The gauges are angularly oriented relative to each other such that the apertures are equally radially spaced about the well casing. As a differential pressure develops from the fluid flow in the reservoir around the well bore, the gauge in the associated apertures most nearly aligned to the direction of flow will give the largest pressure differential reading, and the gauge at 90.degree. will give the lowest reading. A compass or gyroscope, for steel cased holes, is installed at any suitable place to indicate the direction in which the particular gauges are oriented. Means are provided for correlating the readings of the pressure differential gauges with the compass or gyroscope reading whereby the direction of fluid flow and hence the direction of a barrier or fault can be detected. Such information is transmitted to recording devices at the surface.

These and other objects of the invention will become more apparent to those skilled in the art by reference to the following detailed description when viewed in light of the accompanying drawings wherein:

FIG. 1 is a diagrammatic showing of a well, a reservoir, and a barrier or fault in the reservoir;

FIG. 2 shows the sensing apparatus of this invention locked in position within the well bore casing;

FIG. 3 is a cross-sectional view taken along lines 3--3 of FIG. 2; and

FIG. 4 is a cross-sectional view taken along lines 4--4 of FIG. 3.

Referring now to the drawings wherein like numerals indicate like parts, a well bore is generally indicated by the numeral 10, and includes a well bore casing 12 extending throughout the length of the well bore 10 and terminating at one end in a well head 14. The other end is positioned within an oil and gas reservoir 16. For purposes illustrating the method and apparatus of this invention, a barrier or fault 18 is shown within the reservoir 16. FIG. 2 is a view of that portion of the well casing within the reservoir 16 with a portion of the casing broken away to reveal the sensing units 26 and 28 of this invention. A concrete sheath 20 encases the steel casing 12 with both the sheath and casing having apertures in communication with the reservoir to form radially extending passageways 22 spaced around the casing at points to coincide with the openings 24 in the housings of the units 26 and 28. The units 26 and 28 are connected to packer 30. The packers 30 and 40 and steel slips 32 which may be either hydraulically or electrically actuated lock the entire assembly in a fixed position within the casing 12. A compass or gyroscope housing 34 is positioned above the slip. A cable 36 supports the entire assembly and includes electrical conduits leading to the surface.

It is to be understood that both pressure sensing units are identical, therefore, only one, unit 26, will be described in detail. The unit 26 includes a cylindrical housing 38 having an outer diameter less than the inner diameter of the casing 12. A packer assembly 40 encompasses the housing 38 and includes vertically extending radially spaced inflatable ribs 42 which are communicated with each other at the top and bottom through circular conduits 44 and 46. It is to be understood that once the unit 26 is positioned at a predetermined point within the casing, the packing assembly 40 is inflated to hold the unit in a stable and locked position in cooperation with the packer 30 and the slips 32. The housing is enclosed by top and bottom walls 54 and 56 provided with threaded collars 48 and 50 respectively whereby consecutive units may be strung together by means of a threaded nipple such as that indicated by the numeral 52.

As best seen in FIGS. 3 and 4, four differential pressure gauges 58, 60, 62, and 64 are supported by plates 59, 61, 63, and 65 within housing 38. Each of the gauges communicate with a pair of diametrically opposed apertures in the housing 38 through suitable conduits. Gauge 58 communicates with apertures A--A' via conduits 70 and 70' respectively. Gauge 60 communicates with apertures B--B' via conduits 72 and 72' respectively. Gauge 62 communicates with apertures C--C' via conduits 74--74', respectively and gauge 64 communicates with apertures D--D' via conduits 76 and 76' respectively. The apertures A--A', B--B', C--C' and D--D' are equispaced radially around the housing 38, are located in substantially the same plane and communicate with the reservoir through passageways 22 which are substantially aligned with the apertures. The conduits leading from the apertures to the gauges are angulated as necessary to permit readings in the same plane even though the gauges are vertically stacked or otherwise physically positioned within the housing in a manner to accommodate their bulkiness. The particular arrangement of the gauges within the housing is not critical to this invention as long as readings come from points equi-spaced peripherally of the housing. Further, all of the pairs of apertures need not be in the same plane, but the two opposite apertures should be in the same plane. It is more desirable to have all of the apertures in the same plane; but such placement of the apertures may, under certain circumstances, substantially weaken the well casing. The gauges can be of a commercially available type capable of measuring pressure differences in the range of as low as 0.0000001 psi and pressures to 5,000psi.

As mentioned earlier, the housing 34 atop the sensing units contains a gyroscope or a compass for indicating the particular alignment of the respective differential pressure gauges and apertures such that the monitoring equipment on the surface will be correlated to the direction of each gauge. The gauges in FIGS. 3 and 4 are shown with directional indicia thereon.

In accordance with the principles set forth in the first part of this application, the well is completed and production is begun. Following a predetermined production period the well is shut-in. Assuming that the well is not completed near a barrier, the relationship between pressure build-up and 1n (t +.DELTA.t)/.DELTA.t is substantially a straight line. However, in accordance with the theory on which this invention is based, if the well is completed near a barrier such as the barrier 18 shown in FIG. 1, the low pressure point which originally rests at the well casing will move in the direction of the fault 18 as indicated by the arrows in FIG. 1. The result is that the fluid from the greater reservoir area away from the fault will tend to flow past the well casing and toward the newly positioned low pressure point which will be somewhere between the well casing and the fault. Hence, fluid flow is set up across the well casing. A pressure differential will exist between diametrically opposed sides of the well casing in the direction of fluid flow such that the one of the pressure differential gauges 58, 60, 62, and 64 that by the apertures A--A', B--B', C--C', or D--D' is most closely aligned to the direction of flow would give the largest pressure differential reading. The gauge positioned at 90.degree. to the line of flow will give the lowest reading. Hence, since the directional orientation of the respective pressure gauges is known, one must merely employ standard instrumentation to correlate the readings and the orientation of the gauges to indicate the direction of the barrier. Conventional pressure gauges with directional orientation can be utilized according to the preferred embodiment. A standard pressure gauge such as a capacitive manometer may be employed to furnish the desired pressure and direction information. Standard instrumentation such as the type including an AC capacitance bridge can then be used to detect the pressure and direction of the oil field boundary.

Simultaneous or near-simultaneous pressure readings may be taken from the pressure gauges. However, it is not necessary that the readings be taken at exactly the same time if the pressure conditions in the well are relatively stable. If the conditions in the well are stable, then clearly the readings could be taken in intervals.

When first positioning the measuring or sensing units within the well casing, it is important that the pairs of apertures A--A' through D--D' be aligned with the passageways 22 in the steel casing and cement sheath. One method of doing this is to place a radioactive substance in a key passageway 22 and utilize a radioactive sensing device in an associated key aperture in the housing 38; whereby when the two key apertures are aligned and such alignment is so sensed via the radioactive means, the remaining apertures are also aligned.

In a general manner, while there has been disclosed effective and efficient embodiments of the invention, it should be well understood that the invention is not limited to such embodiments as there might be changes made in the arrangement, disposition, and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.

Claims

We claim:

1. A method for locating barriers in underground oil and gas reservoirs comprising the steps of drilling and completing a well in an underground reservoir, operating said well as a given production rate for a predetermined period of time, shutting-in said well, sensing the pressure within said reservoir in at least four equally spaced radial directions from said well, and using differential pressure readings from said pressure sensings to indicate the presence and direction of a barrier in said reservoir.

2. The method of claim 1 wherein said pressures are sensed in eight different directions and wherein said differential pressures are between diametrically opposed directions.